The devastating shortfall in care for young people with mental health problems is revealed today in a leaked government document showing that thousands of families are being left without the support they need.

A letter sent from the Department of Health to senior NHS officials shows that the government will fail to meet its three key targets for children and young people's access to psychiatric care by its deadline of December.

The biggest setback is that for one-quarter of the country there is no emergency help for teenagers suffering a 'psychotic crisis' or severe depression. This '24/7 cover' was seen as essential for vulnerable children and teenagers, who can otherwise become far more seriously ill and sometimes suicidal.

For families whose children have learning disabilities, including autism, the situation is also bleak. Only half of primary care trusts, which commission care from hospitals, can provide access to mental health specialists for these youngsters. The leaked document includes a survey showing that in some areas, such as Kent, there is no mental health service at all for such children.

The third target which cannot be met is for 16 to 17-year-olds to have appropriate care. Many end up in adult psychiatric wards, which can be dangerous.

Cumbria, Lancashire, West Yorkshire and Cornwall emerge as the worst areas for services in a survey the department commissioned. In these mainly rural areas, pressure on NHS cash has made it impossible to set up new psychiatric services specifically for young people.

The letter, sent by Richard Gleave, the department's senior performance management official, to heads of strategic health authorities, says the most recent data on child and adolescent mental health services (CAMHS) shows 'that performance is variable ... and that the NHS is not on track to meet the target by December 2006'.

Gleave warns that if they cannot meet the deadline his unit 'will discuss arranging targeted support from the national CAMHS support service with your colleagues'. This would entail a visit from officials to find out what had gone wrong.

Health minister Rosie Winterton last week said last week there was 'no evidence' that mental health services were being badly affected by the £512m NHS deficit.

But Kathryn Pugh of the charity Young Minds said: 'What the failure to meet targets means is greater distress for the child, for the parents, and inevitably consequences for the schools and social services who then don't have the right support to deal with young people when they get into difficulties.

'If you are a mum with a child who is in great distress, who may be self-harming or profoundly depressed, you need help quickly. Waiting six months for an outpatient appointment isn't going to help.'

Her colleague, Avis Johns, said: 'We know that all around the country there are threats to cuts in services, whatever ministers say. It's astonishing that at a time when children are at their most vulnerable - when their world seems to be collapsing around them - we can't provide quick and appropriate care to keep them out of trouble and relieve them from their illness.'

The charity says there are nearly one million children and young people in Britain who suffer some mental health problem, ranging from anxiety and depression to schizophrenia and general psychotic disturbances. Yet across the country mental health trusts are facing cutbacks. Cambridge's Young People's Service, which has helped thousands back to normal life, faces closure and has launched a fundraising appeal.

A Department of Health spokeswoman said the letter showed that the NHS was focused on providing a comprehensive mental health service for children and families everywhere in England. 'Meeting this target was never going to be easy because of the low baseline of services in many parts of the country,' she said.

But there had been some dramatic improvements already, such as an increase by a quarter in services for children with a learning disability compared with the number a year ago, the spokeswoman added.

· Join the debate: www.observer.co.uk/blog. Listen to Young Minds campaigner Avis Johns talking about the problems of young people and mental health in Britain.